Process for the reduction of sulphur dioxide to elemental sulphur by means of carbon



Dec. 14, 1937. I LEPSQE 2,102,081

- PROCESS FOR THE REDUCTION OF SULPHUR DIOXIDE T0 ELEMENTAL SULPHUR BYMEANS OF CARBON Filed Jan. 14, 1936 2 Sheets-Sheet 1.

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H0555 7' EPSOE WW- Attorney R. LEPSOE 1 PROCESS FOR THE REDUCTION OFSULPHUR DIOXIDE To ELEMENTAL SULPHUR BY MEANS OF CARBON Filed Jan. 14,1956 2 Sheets-Sheet 2 Dec. 14, 1937.

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ROBERT LEPSOE Affornay Patented Dec. 14, 1937, I

' (UNITED. STATES PATE NT TOFFlCE PROCESS FOR, ,THE REDUCTION OF SUL-PHUR DIOXIDE TO SULPHUR BY MEANS OF CARBON Robert Lepsoe, Trail, BritishColumbia, Canada,

' assignor to TheConsolidatcd Mining & Smelting Company of Canada,

Limited, Montreal,

Quebec, Canada, a company of Canada Application January 14, 1936, SerialNo, 59,095 t 9 Claims.

This invention relates to improvements in the process for the reductionof sulphur dioxideto elemental sulphur by means of carbon characterizedin that sulphur dioxide, preheated to a temperature suificiently high tostart the reducing reactions is introduced into a reducing fuel bed andthe temperature of the reducing reactions is at all times positivelycontrolled by admitting additional sulphur dioxide at higher levels ofthe fuel bed thereby increasing .the capacity and efliciency of theprocess to a degree impossible to realize heretofore. i I

In the reduction of sulphur dioxide to elemental sulphur by meansof'carbon, sulphur dioxide is charged into thebase of a reducing fuelbed,

such as coke, in which reducing reactions take place resulting in thereduction of sulphur dioxide to sulphur and other products. Thereactions which take place; however, are exothermic and the temperaturerises toward the top of the furnace where it reaches such an intensitythat I further operation becomes diflicult.

In the reduction of sulphur dioxide to elemental sulphur by means ofcarbon a certain initial temperature is required below which no reactioncan be established. Also the rate of reduction is a function of thetemperature and peratures which would normally result.

I have found that betweenQOO c. and 1400 0.

essentially the following reactions take place:--

There is very little 'carb't' bxysuipiiide (cos) formed above 1000?, C'.and the gas velocity at which I carry out these operations does, notallow suilicient time for any carbon di-sulphide (CS2) formation,althoughzthermodynamic studies"predict the formation of this compoundabove that temperature. a

- The temperature coefilcients of therates of these Reactions (1) and(2) are equivalent since only the time required for ticular ratio of theconcentrations of sulphur the energy of activation (E). in each case isfound to have the same value (which in both these cases is 42,000calories) from the relation ship: I I

' 4.6A log k T where k is the reaction' velocity constant and T is theabsolute temperature in degrees c'entigrade. It follows that the ratioof *the rates of these reactions is constant over a wide temperaturerange. Therefore temperature .will govern dioxide and carbon monoxidewhich would coestablisbing any Darexist at the end of that period; Thusthe higher the temperature the shorter will the time of,

retention become for the reduction, to any par-' ticular extent, ofsulphur dioxide to sulphur and of carbon dioxide to carbon monoxide, andvice versa. It will be apparent that the CO will be formed in increasingamounts as the S02 approaches complete reduction and this is shown inthe following table from the relation of S03 to CO or of S0: to CO+COS(where COS is produced by a secondary reaction with sulphur .vapourpresent according to CO+ S2@COS at temperatures below 1000 C.) in theexit gas from the reduction of S02 by carbon:-

%s0 50 30.20 10 .-s 1 %co 3 7 11 17 23 34 The reaction velocityconstants (lc") of these .Reactions (1) and (2) at the same temperatureare compared in thei'ollowing table over a wide range of temperature andare found from the relationship where P1 and P2 are the initial andiinalpartial pressures of S0: (in Reaction 1) or of CO2 (in Reaction 2) andt" is the time of retention of the gases at any particulartemperature:--

Temp. C. I: so, k 001 i From the above it is apparent that the rate ofReaction (1) is about fifteen times faster than that of Reaction (2) andtherefore the time of retention for the gases will be about fifteentimes shorter in 1) than in the slower reaction (2).

Both rates are approximately doubled for every 50* C. rise intemperature in the lower temperature range.

In considering the development of a hot zone in the furnace it will beseen that while the gases move upwardly through the fuel bed and S: isbeingcontinually reduced, PzSOa and PaCOa will decrease and the increasein the time of retention (t) will'favour CO formation. Reaction (1) isexothermic (heat of reaction =AI-Iso,), Reaction (2) is stronglyendothermic (heat of reaction=AHco,) and the total heat of reduction(Alba) is the sum of the two. I have found that the total heat ofreduction is positive until about 80% of the S0: is reduced, with theresult that the temperature curve reaches a peak near this point. -Thefollowing figures are given for illustration. The initial temperaturehas it appears to be the lowest temperature for practical operation forthe reason that at lower temperatures these reactions are extremelyslow. For other initial temperatures the temperature curves will runpractically parallel to this as the changes due to temperature in AHsoand AI-Ico,

'only-amount'to 3% per'100 c.

Percent Peak Total heat reduction temp. of reduc- 0150s 0. tion Ha Gals.

10 ass out From .these figures it will be seen that in the reduction ofsulphur dioxide by means of carbon, the'conditionsare found to beentirely different to the usual conditions in a .gas producer fur.- naceor in ,any metallurgical furnace as in the former case the hottesttemperature is not generated near the grate or tuyeres but at adistance-higher up in the furnace.

The above data is illustrated in the accompfanying graphs in which. forany percentage of '80: reduction inthefurnace, is shown the temperature(Fig. 1) and thetimefof retention (Fig.

2) resulting from: i

(I) An initial temperature of-900" C. in the primary 80:, no secondary802 being added to the furnace (see curves 1) (II) An initialtemperature of 1050 C. in the primary 80:, no secondary 80: being addedto the furnace (see curves II) (III) [in initial temperature of 1050' C.in the primary 80:, secondary 50: being admitted through two rows oftuyeres (see curves III) Reference to these graphs shows that anincrease inthe temperature of the preheated pri-.

mary sulphur dioxide greatly increases the furnace capacity, and thatthe simultaneous use of secondary sulphur dioxide eliminates thetroubles and wasted time normally resulting from hot zones in thefurnace. A further advantage of my invention is the improvement whichis.

been chosen as 900 C. because made in the heat economy of the processwith regard to the heat required for preheating purposes. I have foundalso that considerably less heat is required for preheating purposes permol. sulphur dioxide reduced in the furnace as a result of usingsecondary sulphur dioxide in conjunction with the preheated primarysulphur dioxide. In this way a smaller quantity of the highly preheatedprimary sulphur dioxide is required per mol. sulphur dioxide reducedthan in the case where no secondary sulphur dioxide is used. These gainsin economy are shown in the following calculations e. g. in Case (3)less energy is required for preheating purposes per mol. 802 reduced,(using preheated primary S02 and cold secondary S02) than in Cases (1)and (2) (using only preheated primary S02). In these cases, reduction ofS0: to the extent of 90% is considered to occur in the furnace, andsince the exit gases will contain 0.10 mol. S02, 0.21 mol. CO, 0.81 mol.C02, 0.45 mol. $2, per mol. original S02,

' they will leave with S0: and CO in approximately the correctproportion of 1 to 2 for further conversion outside the furnace bycatalysis.

Case 1 For preheating 1 mol. primary S0: to 900 C., 9900 cals. will berequired and 0.9 mol. 80: will be completely reduced in 0.71 minute.Thus on the basis of 1 mol. 802 reduced to sulphur, 11000 cals. forpreheating purposes and 0.79 minute time of retention will be necessary.It is found in practice (see Fig. 1, curve I) that when working'with afurnace of adequate capacity a sticky zone occurs-near the top of thefurnace due to development of excessively high temperatures.

Case 2 With 80: preheated to 1050 C. (see Fig. 2, curve 111 90%reduction of 1 mol. 80: can occur in 0.15 minute, i. e. in one quarterthe time taken in the previous case, but in practice, it is found that azone of very bad clinkering (see Fig. 1 curve II) occurs about onequarter of the furnace height above the grate (furnace height referringto the height of the coke bed.) Since 11800 cals. will be required forpreheating 1 mol. primary S0: to 1050-C. and 0.9 mol.-SO: will bereduced in 0.15 minute. therefore on the basis of 1 mol. SO: reduced tosulphur, 13100 cals. for preheating purposes and 0.17 minute time ofretention will be necessary.

Case;

I have found that the clinkering and sticky zones of the furnace can beeliminated and at the same time a high reaction rate can be maintainedby operating the furnace at a high initial temperature and admittingsecondary S0: to the .hot zones.

Thus by preheating the primary SO: to1050 C., and by adding secondary50: Just prior-to the of retention is reduced to about one half of thetime required in'Case 1. For each moi. BOs'preheated to 1050' c. andadded at the'base drum furnace, 0.48 mol. secondary a is added, 81V?".ing a total of 1.48 mols S0: of which or 1.38 mols will be reduced in0.42 m1nute,. (see Fig. 2. curve 111). 0n the basis of 1 12101.80:reduced to sulphur, 8900 cals. will be required for preheating purposesand'the time of retention will be 0.32 minute. The capacity of thefurnace becomes 2%, times greater than that in Case 1, and economy inpreheating is obtained.

In the preceding examples all the figures refer to ideal conditions,that is there are no heat losses and no ash. The figures in actualoperation differ only slightly, the slopes of the curves being the samebut on a slightly lower level.-

In operation the secondary S02 is admitted into the furnace through oneor more rows of tuyeres spaced at various levels throughout the furnaceheight-the furnace height referring to the height of the coke bed. Ihave found, for example, that in operating with an initial temperatureof say 1050 C. two rows of tuyres spaced atone seventh and one third ofthe furnace height are sufficient.

I have found further that if the initial temperature is increased to say1150 -C. clinkering and stickiness can still be avoided and the capacityof the furnace isalmost doubled, but it would be necessary. to addadditional tuyeres, say a third row. In this latter case the meanreducing temperature would be approximately1225 C. as

against 1150 C. which would result from the use of the first mentionedtemperature, i e. .1050" C.

Part of "the sulphur dioxide to be reduced is preheated prior toentering the base of the furnace; the balance of the sulphur dioxide, orsubstantialiy the balance since a small amount may be needed for anauxiliary adjustment of the final gas reaction outside the furnace,enters through a number of inlet ports or tuyeresin one or more rowsabove the base of the furnace in the hot zone area. I

The primary sulphur dioxide may be preheated byany of the well-knownmethods, for example by using an auxiliary furnace such as an electricarc furnace of the Schonherr type, a fire tube furnace or a fire-brickcheckerwork regenerator.

Greater'economy is obtained by preheating cold. sulphur dioxide by heatexchange with the hot exit gases from the reduction furnace prior topreheating in an auxiliary furnace. I prefer,

however, rather than to use an auxiliary furnace,

to develop an initial temperature of approximately 1150 C. in thereduction furnace by mixing regulated amounts of an oxidizing gas suchas air, oxygenated air, or pure oxygen with the sulphur dioxide prior toinjection into the reduction furnace. If necessarythis' mixture is firstpreheated by heat exchange with the. hot exit gases from thereductionfurnace.

- In the event that it is desired to preheat the sulphur dioxide bymeans of an auxiliary furnace-such as a high tension electric arcfurnace of the Schonherr typeit is necessary to add a small amount ofanother gas to the sulphur dioxide in order to give the mixturesufficient conductance; r

For thesake of further economy, the secondary sulphur dioxide may beintroduced at about 400 C. instead of at ordinary temperaturesand isreadily preheated to this temperature by heat exchange withthe hot exitgases. In this manner more secondary sulphur dioxide can be admitted tothe furnace than if admitted cold, for the reason that when thesecondary sulphur dioxide is preheated to any suitable temperature,lower than the temperature of the hot zone below which it is injected,more sulphur dioxide isneeded to produce the required amount of coolingthanin heat would be In this manner a parting from the heated to atemperature 3 the case where the secondary sulphur dioxide is notpreheated.

-It will be apparent from the above-that the hot zones of the furnacecould also be eliminated by admitting cold secondary gas of an inertnature such as nitrogen. or of a partly reactive kind such as CO2,either just prior to the hot zones or with the S0: at the base of thefurnace. In the latter case, while a part of the excessive cold diluentgas, the time for heating the S0: to

absorbed by the-heating of the a reduction temperature would beprolonged and the furnace capacity diminished, and anegligible coolingeffect would result from the endothermic reduction of CO2 by carbon,since this reaction would only produce about 3% CO under suchconditions. In any case, this procedure would cause wasteful consumptionof coke since the heating of the diluent gas by the cooling of the hotzones. would waste heat which otherwise could be utilized. This heat isused in my inventin for heating the secondary S02 locally in thesezonesto a temperature favorable to reduction by exothermic reactionfurther up in the fuel bed. far larger quantity of S02 is reduced thanformerly in the same furnace and gains in the economy. of heat and cokeconsumption are made thereby.

tures and within the temperature ranges as herein disclosed. While theseconditions have been found to give satisfactory results it will beapparent that variations can bemade without descope of the invention.

Having thus fully described my invention, what I claim as new and desireto secure by Letters Patent is:-

1. A process for the reduction -of sulphur dioxide to elemental sulphurby means of carbon which comprises introducing sulphur dioxide, at

a temperature sumciently high to establish the reducing reactions, intoa reducing fuel bed con-' sisting essentially of carbonaceous materialand controlling the temperature of the reducing reactions by admittingat higher levels of the fuel bed additional sulphur dioxide atatemperature below the prevailing temperature of the reacting gases atthe point of admission.

2.. A process for the reduction of sulphur dioxide to elemental sulphurby means of carbon which comprises introducing sulphur dioxide, at I atemperature suflicien-tly high to establish the reducing reactions, intothe base of a reducing fuel bed consisting essentially of carbonaceousmaterial and controlling the temperature of the reducing reactions byadmitting at higher levels of the fuel bed additional sulphur dioxide ata temperature below the prevailing temperature of the reacting gases atthe point of admission.

for the reduction of sulphur diox-'.

3. A process ide to elemental sulphur by means of carbon which comprisesintroducing sulphur dioxide, preabove 900 C., intothe base of a reducingfuel bed consisting essentially of carbonaceous material and controllingthe temperature of the reducing reactions by admitsulphur dioxide at atemperature belowthe' prevailing temperature of the reacting gases atthe I ting at higher levels of the fuel bed additional 4 a temperaturesufficiently high to establish the reducing reactions, into the base ofa reducing fuel bed consisting essentially of carbonaceous material andcontrolling the temperature of the reducing reactions by admitting athigher levels of the fuel bed additional sulphur dioxide preheated to atemperature below the prevailing temperature of the reacting gases atthe point of admission.

5. A process for the reduction of sulphur dioxide to elemental sulphurby means of carbon which comprises introducing sulphur dioxide,preheated to a temperature sufliciently high to establish the reducingreactions, into a reducing fuel bed consisting essentially ofcarbonaceous material, controlling the temperature of the reducingreactions by admitting, at higher levels of the fuel bed, additionalsulphur dioxide at a temperature below the prevailing temperature of thereacting gases at the point of admission and utilizing the sensible heatof the hot gaseous products of the reducing reactions to preheat freshsulphur dioxide prior to its admission to the fuel bed.

6. In a process for the reduction of sulphur dioxide I I the method ofintroducing sulphur dioxide togather with sufficient oxidizing gas intoa reducing fuel bed consisting essentially of carbonaceous '3!) materialto generate a temperature sufiiciently' high to establish the reducingreactions and controlling the temperature of the reducing reactions byadmitting, at higher levels of the fuel bed, additional sulphur dioxideat a temperature 35 below the prevailing temperature of the reactinggases at the point of admission. I

Z; In a process for the reduction of sulphur dioxide to elementalsulphur by means of car- .bon, the method of suiilciently preheating thetions and to elemental sulphur by means of carbon,

sulphur dioxide and introducing it, together with suillcient controlledamounts of an oxidizing gas, into a reducing fuel bed consistingessentially of carbonaceous material to generate a temperaturesumciently high to establish the reducing reactions and controlling thetemperature of the reducing reactions by admitting, at higher levels ofthe fuelbed; additional sulphur dioxide at a'temperature below theprevailing temperature of the reacting gases at the point of admission.I l

8. In a process for the reduction of sulphur dioxide" to elementalsulphur by means of carbon, the method of sufiiciently preheating thesulphur dioxide and introducing it, together with suflicient controlledamounts of an oxidizing gas, into a reducing fuel bed consistingessentially of carbonaceous material to generate a temperaturesumciently high to establish the reducing reaccontrolling thetemperature of the reducing reactions by admitting, at higher levels ofthe fuel bed, additional sulphur dioxide preheated to a temperaturebelow the prevailing temperature of the reacting gases at the point ofadmission.

9. .In a process for the reduction of sulphur dioxide to elementalsulphur by means of carbon, the method of introducing sulphur dioxide,together with sufficient controlled amounts of an oxidizing gas, into areducing fuel bed consisting essentially of carbonaceous material togenerate a temperature sufllciently high to establish the reducingreactions and controlling the temperature of the reducing reactions byadmitting, at higher levels of the fuel bed, additional sulphur dioxidepreheated to a temperature below the prevailing temperature of thereacting gases at the point of admission.

, ROBERT LEPSOE.

